Crusader, not a templar (Though there's less of a line there than you'd think). And he left because I think his crusade ended and he can't live the life he had before leaving for it, so now he's just a sword for hire.
I had a feeling, I just wanted to let you know about the Crusader's backstory since it's pretty good. Pretty much all the classes have stories like that, check Red Hook's website, they've got comics for all the classes' backstories.